The role of the gonadal steroid hormones on the development of the mammalian brain will be studied in long-term, organotypic cultures of the newborn rodent hypothalamus and cerebral cortex. The morphological parameters of differentiation will be analyzed in both living and stained preparations at the light and electron microscopic levels and correlated with biochemical studies of the steroid effects. Information will also be obtained by fluorescence and immunohistochemistry, and 3H-steroid autoradiography. These studies are designed to further characterize the morphogenetic aspects of the steroid responses and the developmental importance of estrogen and androgen in regions of the central nervous system containing high-affinity steroid receptors during their "critical period" of neurogenesis. This unique system can provide new information heretofore unobtainable on the organizational role of these steroids in sexual differentiation of the brain, psychosexual behavior, the reproductive processes proper and brain development in general.